The relationship between placental thickness and gestational age in pregnant women: A cross‐sectional study

Abstract Background and Aims Estimating gestational age (GA) is of utmost importance to assess the condition of the fetus. Incorporating placental thickness and fetal biometrics in estimating GA may improve the accuracy of fetal age estimation. The aim of this study was to examine the relationship between placental thickness and GA in pregnant women referred to Al‐Zahra Hospital's prenatal and emergency clinic in Rasht, Iran. Methods This cross‐sectional study was conducted on pregnant women referred to Al‐Zahra Clinic for prenatal care over a 6‐month period. After obtaining informed consent, data were collected on the date of the first day of the last safe menstruation, average body mass index, and medical and surgical history. Placental thickness was estimated using ultrasound and various clinical information was recorded. The data were then analyzed using Pearson correlation analysis in SPSS software version 21. Results The results showed a significant correlation between GA and placental thickness (p < 0.0001, r = 0.729). Placental thickness increased with increasing GA. There was also a significant relationship between placental thickness and placental location (p = 0.009, r = 0.14). In the posterior position, placental thickness increased by 14% or 0.14. The placental thickness in the posterior position (29.49 ± 0.75) was greater than the anterior position (26.94 ± 10.72). Conclusion The findings of this study suggest that there is a significant increase in placental thickness with increasing GA during the first and second trimesters. Additionally, placental thickness significantly increased in the posterior placental position, as well as in women with high BMI. Therefore, it is recommended that measuring placental thickness should be routine during obstetric ultrasounds.


| INTRODUCTION
In perinatal visits, accurate measurement of gestational age (GA) is critical for distinguishing between normal and abnormal pregnancy, timing screening tests, diagnosing intrauterine growth restriction (IUGR), and deciding on pregnancy termination. To determine GA and usual delivery time, the last menstrual period (LMP) is required. In such studies, the woman should have regular cycles and bleeding volume and not have used birth control tablets in the past 3 months. 1 acteristics are required to estimate GA in fetuses with preterm rupture of membranes. 3,4 As a result, researchers are exploring other diagnostic markers, including thickness. The placental's appearance has received significant attention in recent years because of its connection to fetal growth and reflection fetal health. Midpregnancy changes in the placenta, particularly between 17 and 20 weeks, have been shown to strongly correlate with fetal development and indicate fetal abnormalities. [5][6][7] Ultrasound measurement of placental thickness is a simple and effective criterion for determining GA. The diameter around the head, biparietal diameter, abdominal circumference, and femoral length are all sonographically derived fetal characteristics that are used to date the pregnancy. 8,9 These are commonly used to estimate GA, either alone or in combination. Researchers have demonstrated that placental markers effectively assess children for GA (GA) and that intrauterine growth restriction (IUGR) requires early intervention. 10,11 Estimation of GA to determine placental thickness (PT), encourages obstetricians to regularly assess PT in pregnant women. [12][13][14][15] The combination of these measures with placental thickness may improve GA accuracy. Several studies have reported a relationship between placental thickness and GA in different weeks of pregnancy, but there is no consensus on the specific weeks. 12,16,17 Furthermore, this relationship has varied across different breeds and placenta thickness 16 Given the importance of estimating GA for assessing fetal growth and development 7,18,19 ; abnormal placental thickness has been widely investigated as a diagnostic criterion for evaluating pathological events, 20,21 but it has not been widely adopted in routine prenatal care. This study aimed examine the relationship between placental thickness and GA in pregnant women referred to the prenatal and emergency clinic at Al-Zahra Hospita in Rasht, Iran.

| PATIENTS AND METHODS
A cross-sectional study was conducted using convenience sampling for a period of approximately 6 months at the prenatal clinic of Al-Zahra Hospital in Rasht. Ethical clearance was obtained from the Guilan University Ethical Committee (IR.GUMS.REC.1397.93, and informed consent was obtained from all participants before the commencement of the study. After providing detailed information about the study and answering any questions that the female participants had, their written consent was obtained and they were enrolled in the study. The inclusion criteria included pregnant women with a GA of 11 weeks or more, aged between 20 and 35 years, without any complications, with a reliable date of the first day of the LMP, and no history of surgery or medical disease, and with a body mass index between 20 and 25. The exclusion criteria included patients with an unreliable LMP, chronic diseases such as diabetes, hypertension, or kidney disease, obesity (BMI above 30), multiple pregnancies, fetal-placental abnormalities on ultrasound report, valentous or placental marginal fusion. Additionally, a difference of more than 4 weeks between amenorrhea and uterine height, or the presence of oligohydramnios, or polyhydramnios, were also exclusion criteria.

| Statistical analysis
All data were analyzed using the SPSS software package for Windows version 22.0 (SPSS Inc.). Mean and standard deviation, frequency, and percentage were used to describe the data. relationship between placental thickness and the studied variables was evaluated using Spearman correlation analysis. The significance level in this study was set at less than 0.05.

| Ethical considerations
The study was approved by the Ethics Committee of Guilan

| RESULTS
In this study, the majority of the women were primiparous with a BMI above 25 and an anterior placental location. The obstetrical and clinical characteristics of the participants are shown (Table 1).
Analyzing the correlation between GA based on ultrasound, showed a significant relationship with placental thickness (p = <0.0001) such that the correlation between placental thickness and current GA was 0.729, meaning that with increasing weekly GA, placental thickness increased by 72%, or 0.72. After analyzing the correlation of placental location with placental thickness, a significant relationship was found (p = 0.009). The correlation of placental thickness with placental location was r = 0.148. In posterior position, placental thickness increased by 14% or 0.14. The mean of placental thickness (29.49 ± 0.75) in the posterior part was greater than the thickness in the anterior part (26.94 ± 10.72). After analyzing the correlation between BMI and placental thickness, a significant relationship was also seen (p = 0.029). The correlation between placental thickness and BMI was r = 0.129, meaning that with increasing BMI group, placental thickness increased by 0.12 or 12% (Table 2).
There was a significant correlation between placental thickness and pregnancy trimester, particularly in the first (under 14 weeks) and second trimesters (Table 3). Table 4    The placenta is closely related to both the fetus and the mother, acting as a mirror that reflects their conditions. 22 Thus, in this study, we investigated the relationship between PT and GA, placental position, and maternal BMI to examine and prevent potential risks for the fetus and assist physicians in delivery planning. Our results showed a positive and linear correlation between PT and GA, which could be due to two stages of placental growth during pregnancy: a phase of cellular proliferation lasting up to 36 weeks of gestation and a maturation phase of cellular hypertrophy from 36 weeks to term.
The placenta must also undergo various changes to accommodate the metabolic demands of the embryo. 23,24 This finding is consistent with previous studies, such as Adhikari et al., 17  PT in obesity baboons. 31 However, Durnwald and Mercer's study indicated that a BMI over 30 kg/m 2 did not affect PT. 29 In humans, maternal obesity may lead to various changes in the placenta, including: inflammation, 32 increased villitis, 33 macrophage infiltration, 34 increased placental vascularity, 35 and increased muscularity of placental vessels. 36 In the present study, the increase in PT in obese mothers is likely due to mechanisms. Given the limited on the effect of maternal BMI on PT, further research during clinical research is needed to confirm or disprove this.

| CONCLUSION
In conclusion, our study showed a significant increase in placental thickness with increasing GA in the first and second trimesters.
Furthermore, placental thickness significantly increases in the

CONFLICTS OF INTEREST STATEMENT
The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT
Related data of this project are available upon request.

TRANSPARENCY STATEMENT
The lead author Forozan Milani affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.